The SKS rifle, also known as the Simonov, is a semi-automatic rifle developed for the Soviet army in the 1940's. SKS type rifles have been in widespread use by military services around the world since that time, and are widely available as military surplus for civilian use in the U.S.
The firing mechanism of the SKS is automatically cocked each time a round of ammunition is fired by means of a piston which is actuated by the gas propelling the bullet out of the gun barrel. A small portion of the propellent gas exits the gun barrel through a port penetrating the barrel a short distance from the muzzle and enters a gas tube located above the barrel. The piston is located inside the gas tube and is driven in a rearward direction by the gas against a bolt carrier, momentarily driving the carrier rearwardly against a recoil spring to eject the spent shell casing and cock the hammer. The force of the gas is spent at this point, and the recoil spring drives the bolt carrier forward to introduce a new round from the magazine into the firing chamber.
The standard gas tube produced for SKS rifles is an assembly of several parts. The tube itself is formed in two pieces, the first being a piston tube having a relatively large interior diameter to permit sliding passage of the gas piston head therein, and the second being a rod tube having a smaller inside and outside diameter than the piston tube, with the inside diameter sized to permit passage of an operating rod attached to the gas piston head and the outside sized to permit it to be press fit into the bore of the piston tube. A hand guard, usually made of wood or metal, is attached to the gas tube assembly via end bands to cover the rod tube portion of the gas tube, which becomes hot during continuous firing of the rifle and which is located along the upper surface of the fore end of the stock where a hand may rest while holding a rifle. Standard hand guards comprise three separate pieces: a semi-cylindrical guard, and two end caps or bands which connect the guard to the gas tube.
The overall result of this multi-piece construction is a gas tube assembly that, while functioning adequately in its primary task of converting the energy of propellant gas into piston motion to actuate the firing mechanism, is relatively flexible over its longitudinal axis. This flexibility has several adverse consequences, the most significant being a reduction in firing accuracy of the rifle. The accuracy of firearms can generally be improved by increasing the stiffness or rigidity of the barrel and receiver group components, including the gas system components connected to the rifle barrel.
It is believed by the inventor that the multi-piece construction of the prior art gas tube also leads to the assembly having an increased number of harmonic natural frequencies that are excited when the rifle is fired. During rapid fire these harmonic vibrations may propagate throughout the rifle, decreasing accuracy and possibly causing metal fatigue that may lead to failure of any number of the rifle's components.
Additionally, the multi-piece gas tube has inherently poor heat transfer characteristics. The interface between the two tube patterns impedes heat transfer therebetween and so reduces the overall rate of heat dissipation from the unit.
Another limitation of the standard SKS gas tube assembly is that the hand guard portion cannot be readily detached from the gas tube. The hand guard may need replacement due to damage, or the gun owner may wish to install a different style or color hand guard. It may also be desirable to remove the hand guard from the gas tube assembly for cleaning. Currently, removal of the handguard from the gas tube cannot be accomplished without access to a well equipped machine shop.
The position of the gas tube assembly on the SKS rifle assembly is a prime spot for the mounting of non-standard rifle accessories, such as sighting devices, flashlights, or cameras. The construction of the standard gas tube, however, does not permit the secure mounting of such accessories. No provisions are made on the standard gas tube for mounting accessories, detachably or otherwise.